Polyethylene microporous film is in use for precision filtration film, separators for batteries, separators for condensers and the like. Among these uses, when the film is used as a separator for batteries, particularly as a separator for lithium ion batteries, the film is required to have, in addition to such general characteristics of microporous films as good mechanical strength and permeability, the so-called fuse effect which signifies that when the inside of the battery is overheated the separator is molten to form a film which covers the electrode and breaks the electric current and thereby secures the safety of the battery.
It is known that in the case of polyethylene microporous film the temperature at which the fuse effect manifests itself, that is, the fuse temperature, is in the range of about 130-150.degree. C. Even when the inside of the battery is overheated for some reason, the microporous film melts at the point of time at which the inside temperature reaches the fuse temperature to cover the electrode as a continuous film, whereby the electric current is broken and the battery reaction is stopped. However, when the temperature rise is very rapid the battery temperature may continue to rise even after the fuse effect has been developed with the result that sometimes the continuous film is broken to cause the return of electric current (that is, to cause a short-circuit); this presents a serious problem in respect of safety. Accordingly, the development of a polyethylene microporous film having a high heat resistance which makes it possible to secure the safety of the battery even under such severe conditions has been eagerly desired.
For example, JP-A-4-206257 discloses a method which improves the heat resistance of polyethylene by blending therewith polypropylene, which has a higher melting point than polyethylene. In this method, however, though a certain extent of improvement in heat resistance can be expected in the microporous film, the film readily flows and breaks after becoming molten by overheating even though it contains blended polypropylene, so that a substantial improvement is not attained in respect of securing the safety of the battery. Moreover, this method has a difficulty in that polypropylene is poorly compatible with polyethylene and hence the polymers tend to separate from each other in the microporous film, thereby lowering the film strength.
JP-A-3-105851 discloses a method of improving the mechanical strength of high molecular weight polyethylene by blending therewith a specific amount of superhigh molecular weight polyethylene. Since superhigh molecular weight polyethylene has a considerably high viscosity even after melting, that is, has a good shape-retaining property, the polyethylene microporous film obtained by the disclosed method does not readily break even after being molten, but nevertherless the film does break under severe conditions. Thus, this method does not bring a substantial solution of the problem, like the aforesaid patent disclosure.
JP-A-56-73857, JP-A-63-205048, JP-A-3-274661, JP-A-1-167344 and JP-A-6-329823 disclose methods of improving the mechanical strength, oxidation resistance and heat resistance of polyolefin microporous film by crosslinking the film.
Among them, the invention disclosed by JP-A-56-73857 is directed to a microporous film for lead batteries which contains inert fillers. However, the microporous film has a low mechanical strength of 120 kg/cm.sup.2 and hence is unacceptable for use as separators.
The polyethylene microporous film disclosed in JP-A-63-205048 is a film of a large pore diameter with a maximum pore diameter of 20 .mu.m and can hardly exhibit the fuse effect and moreover involves the risk of short-circuits caused by precipitated metals and crumbled active materials. Thus, it is unsuitable as a separator.
JP-A-1-167344 discloses a method which comprises adding a crosslinking agent, but the polyolefin microporous film obtained by the method shows a low breaking strength of 330 kg/cm.sup.2 or less and hence is unsuitable for use as separators.
JP-A-3-274661 discloses a method which comprises applying the irradiation of ionizing radiation at a relatively small dose of 0.1-10 Mrad to a special microporous film containing the same inert fillers as used in JP-A-56-73856 and to a microporous film produced by the stretching hole-opening method. However, the microporous films thus obtained involve a risk in that, as shown in FIGS. 3 and 4 of JP-A-3-274661, when these polyolefin microporous films are thoroughly crosslinked, the increase of impedance at the time when the fuse effect is to be developed becomes slow, resulting in the delay of electric current breakage. Moreover, according to circumstances, the separator which is in an incomplete state of fusing may conversely become a resistance component and cause the build-up of heat in the battery. Furthermore, these microporous films are unsatisfactory in mechanical strength in view of the needs of the market and also present difficulties in improving the productivity of batteries.
JP-A-6-329823 discloses a method for producing a microporous film which comprises crosslinking a polyolefin sheet, then immersing the sheet in a good solvent for polyolefin to swell the sheet, and preventing the shrinkage of the sheet or stretching the sheet. This method has been devised for the purpose of omitting the step of producing a polyethylene hot solution which has been indispensable in the production of previous polyethylene microporous film, but the disclosure teaches nothing about the heat resistance of the film obtained. Moreover, the method has problems in that since the sheet is prepared without going through a hot solution, stretching at a high draw ratio is difficult to achieve and hence it is difficult to obtain a sheet having a high strength. Moreover, a lot of time is required for sweeling the sheet and hence the method is not practical as an industrial process.